Detection of nuclear material (e.g., nuclear explosive devices, fissile material, and radiological material), both by in situ and remote detection, is important for national security and defense. Conventional nuclear detection technologies are based on the direct detection of nuclear radiation (e.g., the direct detection of gamma (γ) rays). Directly detecting hidden nuclear materials is more difficult. Hidden nuclear materials are typically shielded, with the shielding absorbing most of the nuclear radiation. Accordingly, there is typically a minimal concentration of nuclear radiation, making detection difficult.
For example, assume 10 kg of highly enriched uranium (HEU) is sealed in a lead container having 10 cm thick walls for shielding. The shielding absorbs all alpha (α)-particles, beta (β)-particles, and low energy (e.g., <200 KeV) γ-rays. The shielding may also substantially reduce the rate of higher energy γ-ray and neutron emissions such that passive detection with γ-ray or neutron detectors becomes very difficult beyond a range of 1 meter. Even at a distance of 20 cm from the concealed radiation source, a portable γ-ray detector with sensitivity of 1 μR/h (radiation dose-equivalent with respect to rem (R)) may take over seven hours to generate a single measurement.
An alternative to passive detection is to use active neutron or muon interrogation. In active interrogation, the radiation source is bombarded with neutrons or muons, which introduce fissions in the source. Signatures of the neutron (or muon)-fissions are used to detect the nuclear radiation. The active interrogation method may be unsuitable for use on vehicles and cargo carriers, however, because of the presence of passengers.
The environmental sampling method, started at the International Atomic Energy Agency (IAEA), is generally considered to be the most sensitive and reliable method to confirm the existence or absence of undeclared nuclear materials or nuclear activities. The environmental sampling method, however, uses in situ field operations to collect environmental samples from surfaces of equipment, buildings, air, water, sediments or vegetation. This process is typically very slow and, thus, may not be suitable for nuclear detection applications at airports or boarder ports.